1. Field of the Invention
This invention relates generally to a method for producing a diaryl ether and more particularly concerns a method for producing a di-(mono-or poly-)carboxyaryl ether by the liquid-phase oxidation of a di-(mono- or poly-)alkylaryl ether in a solvent.
2. Discussion of the Prior Art
Over approximately the past two decades, organic compounds in which two or more carboxylic acid groups are bonded to one or more carboxylic and/or heterocyclic aromatic nuclei have become of increasing interest, either as direct components in or as intermediates for synthetic condensation polymer molecules. Some of the polymers containing repeating units derived from aromatic polycarboxylic acids have found broad spectrum utility in synthetic fibers and films, as well as in various types of resin formulations, whereas others of such polymers have been more limited in scope of application, but are not less useful. The more common nuclei of aromatic polycarboxylic acids that form polymers that are useful for these purposes include certain simple and more complex bridged aromatic ring systems. One important member of the group involving bridged aromatic ring systems can be pictured most simply by the formula: Ar--O--Ar.sup.1, where the groups, Ar and Ar.sup.1, represent the same or different cyclic aromatic nuclei. In some cases, the aromatic nuclei of such polycarboxylic acids will contain one or more additional ring substituents, such as amino, nitro, halogen, hydroxyl, cyano, sulfonyl, and the like groups. These additional groups do not participate in the principal polymer-forming reactions, but they can be desirable molecular constituents, either because of the properties they impart to the polymer or because they render the initial polymer molecules susceptible to modification by further reaction.
The aromatic polycarboxylic acids used in polymers generally are produced by subjecting aromatic compounds having a plurality of appropriately positioned alkyl substituents on the aromatic ring or rings to oxidation processes. A number of processes have been reported for oxidizing alkylated aromatic compounds using an oxygen-containing gas and a metal-containing catalyst In particular, several prior art patents have disclosed methods for the oxidation of alkylated diaryl ethers. For example, U.S. Pat. No. 2,959,613 discloses a process for the liquid phase oxidation of alkylated compounds of aromatic character or their closely related oxygenated derivatives at 50.degree.-300.degree. C. and at atmospheric or superatmospheric pressure of up to 200 atmospheres, and in the presence of a catalyst having manganese, cobalt and bromine components and additionally in the presence of cations--for example, any basic ions but preferably alkali metals or alkaline earth metals--corresponding to a concentration of from 0.25 to 0.00025 gram atom per gram mole of total organic compounds. Of particular relevance, the patent also discloses that hydrocarbons of aromatic character substituted by at least one alkyl, haloalkyl, or closely related oxygenated derivative of an alkyl or haloalkyl group and by "at least one polar group resistant to oxidation selected from: halogen, --SO.sub.2 NR'R"(R'R"=alkyl, aryl, or H), --OR(R=alkyl, aryl), --NHCOR(R=alkyl aryl or H), --OCOR(R=alkyl, aryl, or H), --SO.sub.3 R(R=alkyl, aryl or H), --CONR'R"(R=alkyl, aryl or H), NR'R"(R=alkyl aryl), benzoyl, substituted benzoyl or alkyl carboxylic ester" can also be oxidized to the corresponding carboxylic acids. However, this patent contains no disclosure of a specific, suitable compound containing a polar --OR group where R is an aryl group, that is resistant to oxidation.
U.S. Pat. No. 3,012,038 discloses a process for the liquid-phase oxidation of alkyl or haloalkyl aromatic compounds or heterocyclic compounds of aromatic character or their closely related oxygen-containing derivatives to form carboxylic acids, in the presence of a catalyst having cobalt manganese and bromine components and in a reaction zone which presents surfaces of titanium, tantalum or hafnium. The only ether or sulfide that is disclosed as a starting material is beta, beta'-dichlorodiethylether.
U.S. Pat. No. 3,406,196 discloses a process for the oxidation in two stages of polyalkyl-substituted aromatic compounds to polycarboxylic acids with molecular oxygen in the presence of a catalyst having cobalt, manganese, and bromine components and in the absence of a foreign organic liquid reaction medium. The second oxidation stage is conducted at a temperature that is at least 25.degree. C. higher than the temperature employed in the first oxidation stage. Suitable polyalkylated aromatics include bridged aromatic ring systems pictured most simply by the formula Ar-Ar' and Ar''-X-AR''' where Ar and Ar''' represent like or different cyclic aromatic nuclei and X represents --O--, --SO--, --SO.sub.2 --, --CO-- or --[CRR']--.sub.n where R and R' represent hydrogen and organic radicals and n is a positive integer of at least one. A specific example of a suitable polyalkylated aromatic ether (where X is oxygen) is not disclosed therein.
Tanger, U.S. Pat. Nos. 4,323,692 and 4,401,828 disclose the oxidation of the methyl substituent in ##STR1## to ##STR2## wherein R is OR',SR', NR'R' wherein R' is hydrogen, alkyl, substituted alkyl or alkenyl, and wherein X is hydrogen or halogen. The disclosed process employs a cobalt catalyst, a bromide promoter and a hydrogen peroxide activator.
Imamura, U.S. Pat. No. 4,220,605 and the divisional thereof, U.S. Pat. No. 4,272,634, disclose the oxidation at 30-200.degree. C. to an alcohol or aldehyde of the methyl substituent of a toluene derivative of the formula ##STR3## where R is a hydrocarbyl group with 1-20 carbon atoms which may carry one or more substituents which are described as being inert to the oxidation reaction, and n is an integer of 1 or 2. R may be an aryl group such as phenyl, tolyl, xylyl, ethylphenyl, n-propylphenyl, isopropylphenyl, butylphenyl, naphthyl groups and aralkyl groups such as benzyl and phenethyl groups. The catalyst employed in the oxidation reaction contains a soluble cobalt salt and a bromine ion-supplying substance. Water formed during the reaction is removed from the reaction system. Carboxylic acids are disclosed as by-products whose formation is inhibited under the conditions employed in the claimed method.
Similarly, British Pat. No. 1,546,397 discloses the preparation of formylated phenoxy compounds by the liquid-phase oxidation of methylated phenoxy compounds with an oxygen-containing gas above atmospheric pressure in the presence of a lower fatty acid or anhydride and at least one soluble salt of cobalt, manganese, chromium or nickel.
Back, U.S. Pat. No. 3,453,324 discloses the oxidation of 4,4'-bis(hydroxymethyl)-diphenyl ether to 4,4-bis(carboxy)-diphenylether under essentially anhydrous conditions at a temperature as high as the reflux temperature of the reaction mass and with a specific catalyst system which is described as having as its essential feature that it consists essentially of a cobalt salt of a normally liquid fatty acid and a bromide ion in concentrations which provide at least about one atom of cobalt for every atom of bromine present in the system.
British Pat. No. 951,192 discloses a method for oxidizing aralkyl compounds which may contain oxygen or sulfur, for example, polyalkylaryl ethers and sulfones, such as p,p'-dimethyldiphenyl ether to p,p'-diphenylether dicarboxylic acid, in the presence of a catalyst which comprises cobalt, bromine and a carboxylic acid, which can also be the solvent, and preferably under substantially anhydrous conditions. The catalyst system is described as being so unique and specific that the omission or substitution of one component either totally stops or substantially impedes the reaction.
Similarly, Japanese Kokai-Tokkyo Koho JP 86 63,634 discloses the oxidation of dimethyldiphenyl ethers with molecular oxygen in a lower fatty acid solvent and in the presence of a catalyst containing cobalt and bromine.
One of the difficulties with such prior art oxidation procedures lies in their inability to provide an essentially pure product without expensive and time-consuming separation procedures. Moreover, where polyalkyl derivatives are employed as reactants, the reaction sometimes appears to proceed with the oxidation of one of the alkyl radicals to the exclusion of the remainder. Such byproducts frequently tend to possess solubility properties similar to those possessed by the desired product making separation and purification of the desired product expensive and time-consuming in commercial operations. A further difficulty encountered in such reactions involving the formation of intermediate oxidation products resides in the fact that conditions and catalyst materials which function most efficiently at one stage of the reaction may become less efficient for oxidation at another intermediate stage.